Preheat the oven to 350°; butter a 13x9 inch baking dish.
Bring 6 quarts of salted water to boiling; add in the pasta and cook, stirring occasionally, 9-10 minutes or until al dente; drain.
In a large saucepan, combine the cream, milk, cayenne, and nutmeg over medium-high heat; reduce by half (the author says about 15 minutes but I don’t know); once the mixture starts to simmer, stir to break the layer of foam so that it will not boil over.
Lower heat to medium-low; add in the American and Monterey Jack cheeses; whisk until well blended.
Season to taste with salt and pepper.
Add the penne to the sauce; stir vigorously (taking care not to break the pasta); this will release starch and help thicken the sauce.
Transfer mixture to the baking dish; combine the panko, parsley, and parmesan; sprinkle over the pasta.